It is still posible for you to make a 5 gallon all grain biab with your 5 gallon pot. You will need to do a high gravity partial boil and top off the water in the fermentor (or BK after the boil). This method is referred to as maxi biab if you want to do some searching.
I did this last week with roughly 13 lbs of grain in 3.75 gallons of water. Make sure your grain bag is large enough. I ended up with almost 3 gallons of 1.070 wort. My efficiency sucked but it worked. I did not sparge either.
You will need to scale up the malt in your recipe his way though.
Olympian DIPA - 8%, Robust Porter - 5%, Red Ale - 4.7%, ESB - 5.4%, Abbey Dubbel - 6.8%
In the fermentor:
Citra IPA, Dartmouth Pilsner, Belgian Fruitcake